Yes, this is normal. A department might have 10 spots for new students. They might know that generally, 50% of the people they admit accept, so they may choose to make 20 offers. However, some schools don't want to risk having too many students so they might only admit between 10 and 20 depending on how sure they are that an applicant will take the offer. For example, if 5 of the top 10 applicants told them they will accept for sure, then they won't make too many offers above 10.

And the reason why they ask is that if they wait until people turn them down, they will miss out on good candidates (and good candidates will miss out on their chance at this school). It could be that candidate ranked #16 for this school would love to attend but if they only make 10 offers at first and wait until these 10 decline, then #16 might already took an offer elsewhere.

Sometimes school ask it indirectly by asking something like, "what other schools are you considering?", which makes a lot of applicants nervous because they think it's a trick question and if you say something like their rival school, they will be upset. This is highly unlikely to be true because schools are just probably trying to figure out how many offers to make. Another way schools ask this question is "how do we compare to other schools, in terms of choice?" or something like that.

So in your situation, it's good that the school is up front and letting you know why they are asking so you don't have to worry about it. My advice in your situation is to answer honestly. You already have an offer and you will be attending some graduate school and be part of the academic community. So, it would be a good idea to act in good faith and help them with their request.

It sounds like you don't know for sure how you feel about the schools yet and you probably would like to attend their open house. So you can say something like that. At this stage, I recommend being honest about how you feel about the school. For example, you might want to say something like:

#1: "Your program is my first choice right now but I would like to delay any decision until after I have attended the open house. I look forward to meeting everyone soon!"

#2: "Your program is one of my top choices. I won't be able to make a final decision until I have visited my top choice school. I look forward to meeting everyone soon!"

#3: "I am still waiting to hear from many/some/a few other schools so I am not sure how likely I would accept this offer yet. I am definitely interested in your program and look forward to visiting soon."

#4: "I am no longer interested in this offer. Thank you for the opportunity and consideration."

These are just rough drafts that you probably would want to edit into something more specific and fitting for your scenario. I hope options #1, #2, and #4 are clear in their purpose. #3 is the hardest one to write, I think, and it would be for a case where School A is your safety school and you haven't heard back from others yet, or it's a middle-tiered school while you're still hoping for one of your higher choices. In this version of the #3 reply, I decided to emphasize your interest in the program but refrain from saying it's (one of) the top choices. Hopefully, this communicates that it's one of the second tier choices without directly saying so (I assume the admissions committee receives tons of replies between #1 and #4 and can calibrate this type of responses vs what others said in the past. However, you can also choose to be more direct if you wish!